The Real Supersonic
Go to stage four and collect 50 or more rings. Press A, B, and C at the same time. The music will change. You will be a golden Yellow and twice as fast. You can jump twice as high too. I would collect 70 rings if I were you. Once you use this code, every 1 or 2 seconds makes you lose a ring. When you lose all of your rings or beat the boss to go to Act 2, you will become normal again.

View Animations
Enter debug mode and press B+C. Then Sonic and Tails will show-off their moves!

Different Battle Music
Just before you fight Robotnik in the Hydrocity Zone, act 2, the boss music will come. Keep Sonic's head under the shallow water until he is about to drown, then pop out of the water. You will hear the boss music from Sonic and Knuckles.

Another (Harder) Level Select code
First, you must have a Sonic 2 cartridge. Take it and enter Sonic 2 level select, which are sounds 19, 65, 09, 17, return to title screen, hold A+START. Once in Sonic 2 Level Select, hold A, remove the cartridge with the Genesis on, Insert Sonic 3, and press Reset. Now, release A and press Down twice at main menu.

Change Identities
Enter Debug mode, start a level that has a special stage ring, choose a sprite of your choice, enter the ring as that sprite and hit B twice, as soon as you leave that zone, you'll be that sprite for the rest of the level.

Debug Mode
Enter the level select code, then press A + START and choose your level. Once the action starts, press C to create an item, B to toggle between items, and A to select items.

SuperSonic
Press START to play, or press A+START to play with SuperSonic- You'll notice the score is scrambled. Collect 50 rings and double jump for SuperSonic.

Level Select
Switch on the machine, and when the SEGA logo appears and Sonic jumps forward, quickly press:
UP, UP, DOWN, DOWN, UP, UP, UP, UP, UP. If you've done it right, you'll hear a ring sound. It's VERY difficult to get working, so keep resetting and trying again if it won't work. If it does work, on the title screen, highlight "1 PLAYER" and press UP, a new option called SOUND TEST appears. Press start to enter it and select which level you want to play from.